I think it's not so much a question of deliberate overspeccing, but more that each of these missions is its own prototype. You're asked to design something to

I think it's not so much a question of deliberate overspeccing, but more that each of these missions is its own prototype. You're asked to design something to do a small part of the task, but you won't get a chance to try again if it goes wrong. You really really don't want your part to be the reason the mission fails. And you don't get to test your part in the real deployment environment and find out everything you need to know before your design the production part. So you have to design for every eventuality you can foresee, and then add some margin for the events you cannot foresee. So even if the spec is exactly right, to ensure you satisfy the spec first time with the uncertainties of the production environment, you end up producing a part that has as much margin for error as you can get away with in the mass and financial budget. Everyone involved does the same thing, because no-one really knows what the production environment will be like, and no-one wants to be the reason the mission fails. And so you end up with a spacecraft that is as overengineered as possible given the budget, even if it isn't specced that way.